Not applicable.
The present invention relates to an improved apparatus and its use in a method to control sample motion in magnetometers, such as vibrating sample magnetometers, where an oscillatory sample motion is required.
Vibrating sample magnetometers (VSM""s) have become standard equipment for measuring the magnetic moment of materials. Detailed descriptions of the VSM first appeared with the pioneering work of S. Foner (xe2x80x9cVersatile and Sensitive Vibrating-Sample Magnetometerxe2x80x9d, Review of Scientific Instruments, Vol. 30, pp. 548-557 (1959); see also U.S. Pat. Nos. 2,946,948, 3,496,459, and 4,005,358).
Conceptually, the vibrating sample magnetometer is very straightforward. The sample to be measured is vibrated in a magnetic field that is typically generated by laboratory electromagnets or superconducting solenoids. Inductive pick-up coils are mounted near the sample. These coils are an assembly of wire wound coils with varying geometries depending upon the specifics of the system and what is desired in the measurement. As the magnetized sample vibrates, a voltage is induced in the pick-up coils (Faraday""s Law) that is directly related to the magnetic moment of the sample. With proper calibration, this voltage output directly yields the magnetic moment of the sample. Magnetic moment is measured in either electromagnetic units (emu) in the Centimeter-Gram-Second (cgs) system of units or in ampere-meters squared (A-m2) in The International System of Units (SI).
Generally the quality of a VSM measurement depends upon the proper integration of all components of the VSM: sample drive, pick-up coils, magnetic field generation, and signal detection electronics. Since the publication of the first description of a VSM, there have been a considerable number of publications describing improvements in all aspects of the VSM. The present day VSM has evolved into a very sophisticated instrument featuring state-of-the-art detection schemes, control, and technology. This invention relates to the drive method used to vibrate and control the sample.
One of the most common sample drives used in VSM""s is an alternating current (ac) driven voice coil drive, similar to what is used in a common loudspeaker. In fact, Foner""s first detailed description referenced above shows a loudspeaker to which a rod and sample are attached. Since the VSM output depends directly upon the stability, control, and isolation of the sample motion, more elaborate voice coil drives have evolved through the years. These have included drives using one or more of the following: phosphor bronze springs for mechanical integrity, a means to feedback drive signal information to control the vibration amplitude, and vibration damping schemes to minimize vibration coupling to the detection coil system. Other methods have been used to vibrate the sample, such as motors with camshafts, but the voice coil drive has remained the most widely used due to its simplicity and ease of use.
Although different implementations of the mechanics surrounding the voice coil continue to be employed by users, control of the ac drive amplitude using an ac feedback signal has become standard procedure to maintain precise control of the sample oscillation. A variety of feedback schemes to monitor the sample motion have been used. In one method, a parallel plate capacitor is formed between one plate rigidly attached to the drive housing and a second plate attached to a sample rod that is rigidly connected to and supports the sample. As the sample and sample rod move, the variation in capacitance is used in a feedback loop to control the sample drive oscillation. Another feedback scheme uses a permanent magnet mounted to the sample rod well outside of the magnetic field and rigidly mounted inductive pick-up coils. As the rod vibrates, the voltage induced in the pick-up coils by the vibrating permanent magnet is used in a feedback loop to control the sample drive. Operating frequencies for the VSM voice coil drives typical fall within the range of 30 to 100 Hertz (Hz).
While much work has been done in the past, there are a number of shortcomings with existing drive technologies. For example, current mechanical assemblies used in a VSM drive can require warm-up times of several hours to achieve their specified stability. Further, such assemblies can show drifts associated with environmental changes such as temperature and humidity. Also, any non-linearities in the ac behavior of the feedback sensor or voice coil related to the positioning of the elements, whether capacitor plates, permanent magnets, voice coil components, etc., can cause variations in the VSM output.
Increasing technological demands for improved magnetic measurements are requiring improvements in all aspects of magnetometer performance. Illustrative embodiments of the present invention overcome the disadvantages noted above for existing drive technologies by providing an improved device and methodology for voice coil drives suitable for these magnetometer applications.
One aspect of an illustrative embodiment of the present invention provides an apparatus and a method for improved control of a voice coil sample drive through the use of both ac and direct current (dc) voltage control that results in better overall stability and performance for a VSM.
The apparatus of the illustrative embodiment present invention comprises a VSM sample drive which uses a feedback sensor that provides not only ac amplitude information on the sample movement, but also absolute position information. This feedback is used in proportional/integral (PI) control loops to control both a dc and an ac drive to the voice coil. The dc drive signal maintains precise positioning of the voice coil/feedback sensor arrangement while the ac drive signal provides the control required for the oscillatory sample motion.